I don't know about investment value, but some of the sets are rather nice. I like the 73 set in particular because of its imaginative photography. It seems that Topps got really complacent with its mid-to-late 60s sets, knowing that it had no competition. I don't know what changed in the early 70s, maybe just a new design team?, but they started doing some very creative things.
Here's
Vida Blue from 73.
Bench is fun, not often that you can't even see a player's face on his baseball card.
Some
really great sliding shots.
It's a fun and refreshing set. One of my favorites.